The present invention generally relates to a method for selective removal of cadmium (Cd) from a feed solution also containing other metals such as nickel (Ni) and/or cobalt (Co), utilizing a thiourea based ion exchange resin, and to a method for eluting cadmium adsorbed on the resin.
Leach liquors produced in chemical and hydrometallurgical processes frequently contain metal impurities which could adversely affect the commercial marketability or desirability of the recovered metal(s).
For example, pressure oxidative leaching (POL) of VBN (Voisey's Bay Nickel) concentrate, or the POL process, produces nickel solution which contains several (typically 2-4) mg/L of cadmium. If this cadmium is not remove from the solution prior to nickel electrowinning, the nickel cathode product will contain ˜50-100 ppm cadmium. This level far exceeds the cadmium level of <5 ppm typically found in commercially available plating grade nickel.
For a more compelling discussion of POL, see U.S. Pat. No. 6,428,604 to Kerfoot et al.
Cadmium has been very low in plating grade nickel because the feed materials, such as nickel mattes or laterite sourced nickel-cobalt sulfide, typically have negligible levels of cadmium.
In hydrometallurgical base metal refining, cadmium removal is commonly practiced in zinc refining, where it is removed by cementation with zinc dust. This is possible because cadmium is a more noble metal than zinc. Since cadmium is less noble than nickel and cobalt, it is not possible to cement it using zinc dust from nickel and/or cobalt solutions—unless significant nickel and/or cobalt cementation can be tolerated. Removal of cadmium to very low levels from nickel or cobalt solutions by precipitation as a metal sulfide, by using hydrogen sulfide H2S or sodium hydrogen sulfide NaSH), is possible but with co-precipitation of nickel or cobalt. For example, a precipitation circuit operated for cadmium removal by H2S from a concentrated (about 90 g/L) cobalt solution, resulted in precipitation of cadmium as sulfide, however, the loss of cobalt was very significant. Also, cadmium sulfide is relatively unstable and the precipitate tends to rapidly redissolve.
Cadmium can be removed by solvent extraction from nickel or cobalt sulfate solutions using organophosphorus acid extractants. However, if the solutions also contain increasing amounts of chloride, the extraction effectiveness for cadmium is progressively reduced.
For these reasons, cadmium removal from a nickel or cobalt sulfate solution, which may contain chlorides—such as the VBN POL nickel solution, by solvent extraction with organophosphorus acid extractants, is minimal.
Although it is possible to achieve higher cadmium extraction in such SX circuits, this will also result in significant coextraction of nickel (and cobalt)—which is again undesirable.
Nickel/cobalt co-extraction is undesirable because it will represent a significant loss from the Ni/Co refining process and/or an increase in the in-process nickel recirculation load. Furthermore, it will result in lowering the Ni and Co levels in the product solutions. These solutions are also the feeds to the respective electrowinning operations for Ni and Co and the efficiency of the electrowinning (and of the entire metal refining operation) depends on maintaining high Ni and Co tenors to electrowinning.
Therefore, a successful method for Cd removal from nickel solutions must be highly selective for Cd over Ni and Co.
Two types of ion-exchange resins are known to be capable of selectively removing cadmium.
The first type is a thiol (having —SH functional group) based resin. An example of a commercially available resin is Rohm & Haas' Amberlite® GT73. Although this type of resin has been developed and is commercially used for mercury removal, it is known that it also removes cadmium. Cadmium removal using a thio resin is described in Atia et al, “Synthesis of amine and thio chelating resins and study of their interaction with zinc(II), cadmium(II) and mercury(II) ions in their aqueous solutions,” Reactive & Functional Polymers, 56(1), pp. 75-82, 2003, and in Saha, et al., “Sorption of trace heavy metals by thiol containing chelating resins”, Solvent Extraction and Ion Exchange, 18(1), pp. 133-167, 2000. The main disadvantages of these resins are that they are prone to oxidation and lose metal loading capacity over a few loading cycles, and the elution of the loaded metals (including cadmium) require strong acid (such as hydrochloric acid) and thiourea.
The second type of ion-exchange resins is a thiourea based resin. An example of a commercially available resin is Lanxess' Lewatit® TP 214. The thiourea resin has known applications for mercury, platinum metals, gold and silver. Cadmium removal using a thiourea based resin is described in the English language abstract of Kobayashi and Oseto, “Elimination of cadmium ion in waste water by adsorbents,” Mizu Shori Gijutsu, 19(8), pp. 727-33, 1982, and in Zuo et al, “Selective binding of mercury to thiourea-based coordinating resins,” Reactive & Functional Polymers, Vol. 27, No. 3, pp. 187-198, Nov. 1, 1995.
Using thiourea as an eluant for cadmium has several disadvantages: a) using thiourea solution runs the risk of leaking thiourea into the cadmium-free nickel electrolyte which is highly undesirable as thiourea has a very negative impact on the quality of the plated nickel in the downstream nickel electrowinning operation; b) Cd elution requires high consumption of thiourea as the Cd concentration in the thiourea eluate has to be kept low in order to maintain effectiveness of the elution; c) the large volume of cadmium thiourea eluate has to be oxidized (as thiourea is cancerogenic) before disposal, and this adds further to the already high cost of the elution with thiourea.
Moreover, none of the aforementioned published studies disclose using an acidic chloride solution for resin activation, followed by water elution, for elution of loaded Cd from a thiourea resin, thus avoiding the use of thiourea solution.
It is therefore particularly advantageous to develop a method for preferentially removing cadmium from a feed solution also containing, for example, cobalt and/or nickel and a method for eluting cadmium from thiourea based resins, which do not have the disadvantages associated with known prior art processes intended for a similar purpose and which do not utilize thiourea as an eluant for cadmium.